Plenarvorträge - DPG-Tagungen

Dynamik und Statistische Physik Donnerstag
setup.
DY 46.65 Do 16:00 Poster D
Turbulente Geschwindigkeitsprofile über rauhen Ebenen —
•Nikolaus von der Heydt und Irmgard von der Heydt
— Institut für Umweltphysik Göttingen - Physik zum Leben -
Landolfshausen
In atmosphärischen Luftströmungen wurden in Höhen bis zu 100 m
erhebliche Abweichungen von der logarithmischen Höhenformel u(z) =
w z ln( ) festgestellt: Die Anpassung der vermeintlich geländetypischen,
κ z0
sogenannten Rauhigkeitslänge z0 ergab an ein und demselben Ort in
Abhängigkeit von der Schubspannungsgeschwindigkeit w = � σ
ρ (zwischen
0, 2 und 1 m/s) Werte von 1 mm bis 1 m. Ausgehend von neueren
Ergebnissen der Turbulenzforschung zeigen wir, dass Prandtl’s Ansatz
für die Wirbelzähigkeit νPr = κwz, der zur Log-Formel führt, verbessert
werden kann, und leiten damit eine DGL für u(z) her, deren Lösung für
z >> ν (ν kinemat. Viskosität) selbstähnlich ist und mit den Messdaten
w
übereinstimmt.
DY 46.66 Do 16:00 Poster D
Heterogeneous Nucleation of charged sphere suspensions on
shear induced precursors — •Thomas Palberg, Andreas Stipp,
Ralf Biehl, Hans Joachim Schöpe, Jianing Liu, and Thorsten
Preis — Institut f. Physik, Universität Mainz, Staudinger Weg 7, 55128
Mainz
Growth of oriented single crystals may be obtained from shear melts
of colloidal particles after nucleation at the container walls. Using different
microscopic and scattering techniques we here studied precursors of
such nuclei induced by steady shear. In all cases we observed shear layer
structures which were oriented with their densest plane parallel to the
container wall and their densest packed direction parallel to the flow. A
strong correlation between the modes of motion and the type of interlayer
registering is observed. Depending on the shear geometry used, the
equilibrium phase, the strength and the range of interaction such layer
structures either disappear upon increase of the shear rate or grow to
macroscopic objects. Stop of shear may lead to an interesting competition
between the decay of these usually meta-stable structures in favour
of the shear melt and their stabilisation through coverage by an epitaxially
growing wall crystal. In the latter case the shear layer residues act
as pre-structured substrate.
DY 46.67 Do 16:00 Poster D
Lattice-Boltzmann scheme for the dendritic growth in presence
of convection — •Dmitry Medvedev and Klaus Kassner — Institut
fuer Theoretische Physik / Computerorientierte Theor. Physik, Ottovon-Guericke-Universitaet
Magdeburg, Postfach 4120, D-39016 Magdeburg
Dmitry Medvedev, Klaus Kassner, Otto-von-Guericke Universität
Magdeburg
A combined phase-field/lattice-Boltzmann scheme is proposed to simulate
dendritic growth from a supercooled melt.
The phase change part of the problem is treated with the phase-field
approach of Karma and Rappel, whereas the flow of the liquid is simulated
by a standard lattice-Boltzmann-BGK (LBGK) method with interactions
with solid and thermal convection incorporated. To simulate
conductive and convective heat transfer we use the multicomponent LBE
method. The step of flow simulation can be dropped out in the case of
purely diffusional growth.
Test simulations without convection reproduce the dendrite tip velocity
of the exact solution.
Depending on the level of anisotropy, dendrites or doublons were obtained
in the simulations.
Dendritic growth in a shear flow was simulated for different flow velocities,
as well as the growth with natural thermal convection with different
orientations of the crystal in the gravitational field.
DY 46.68 Do 16:00 Poster D
Kinetic roughening of laser deposited polymer films: Crossover
from single particle to continuous growth — •Jörg Hachenberg
1 , Christoph Streng 1 , Erik Süske 2 , Sebastian Vauth 1 , Stefan
G. Mayr 1 , Hans-Ulrich Krebs 2 , and Konrad Samwer 1 — 1 I.
Physikalisches Institut, Universität Göttingen — 2 Institut für Materialphysik,
Universität Göttingen, Tammannstr. 1, 37077 Göttingen
Thin film samples are prepared by pulsed laser deposition (PLD) of
Bisphenol-A (BPA) polycarbonate targets at room temperature and un-
der UHV conditions. Atomic force microscopy (AFM) is used to investigate
the development of the surface roughness as a function of the
thickness and the deposition angle.
We find that the roughness increases proportional to the film thickness
by the growth exponent of 1/4 between 20nm and more than 4µm.
This is interpreted within computer simulations and theoretical models.
Similar results have been shown in amorphous metallic systems, but
on different scales. For lower film thicknesses than 20nm we observed a
crossover to even steeper slopes where the particle character dominates
the roughening and a continuous description is no longer valid. Additional
investigations under variation of the laser fluency have been performed.
This project is supported by the DFG, SFB 602, TP B3 and GRK 782.
DY 46.69 Do 16:00 Poster D
Shape of mounds and scaling behaviour in unstable epitaxial
crystal growth — •Markus Walther, Sebastian Weber,
and Michael Biehl — Institut für Theoretische Physik,Universität
Würzburg, Am Hubland, 97074 Würzburg
We study the influence of an Ehrlich-Schwoebel like barrier at corners
in the diffusion of adatoms along step edges. The presence of an infinite
barrier determines the shape of mounds in unstable growth and the
coarsening dynamics of the surface.
The investigation is realized by two different methods of Monte Carlo
simulations leading to similar results: Kinetic Monte Carlo simulations
and a single particle representation. We investigate the effect of a finite
barrier and its role in a dynamical phase transition with respect to surface
morphology and scaling behaviour.
DY 46.70 Do 16:00 Poster D
Comparison of Three-dimensional Simulations of Dendrites
with Experimentally Grown 3D Xenon Crystals — •Herman M.
Singer and Jörg H. Bilgram — Laboratorium für Festkörperphysik,
ETH Zürich, 8093 Zürich (Schweiz)
Dendritic growth from a pure substance into supercooled melt was simulated
in two- and three-dimensional calculations of two different phase
field models ([A.A. Wheeler, B.T. Murray and R.J. Schaefer, Physica D
66, 243 (1993)] and [A. Karma and W.-J. Rappel, Phys. Rev. E 57, 4323
(1998)]). By means of large scale parallel molecular dynamics simulations
for a Lennard-Jones potential, estimates for the anisotropy of interfacial
free energy for our experimental model substance were calculated. In our
in situ experiments three dimensional xenon crystals during free growth
can be observed. Apart from dendrites, doublon and seaweed morphologies
as well as morphology transitions are observed. Those structures can
be simulated by using special initial conditions. A morphology diagram
for 2D and 3D simulations is presented and compared with the results of
experiments and analytical studies.
DY 46.71 Do 16:00 Poster D
Analysis of Solidification Dynamics by Wavelet Transformation
Techniques — •Oliver Wittwer and Jörg H. Bilgram — Laboratorium
für Festkörperphysik, ETH Zürich, 8093 Zürich (Schweiz)
Complex structures develop during solidification processes. Changes
in growth morphology can be initiated in our experiments by changing
the temperature distribution in the environment of a growing crystal [I.
Stalder and J.H. Bilgram, Europhys, Lett. 56, 829 (2001)]. These transitions
have gained interest since they have been discovered in metallic
samples [K. Dragevski et al., Phys. Rev. Lett. 89, 215502 (2002)]. To
study transitions in growth morphology and to monitor quasi stationary
growth of dendrites we apply wavelet transformation techniques. We are
able to detect changes in growth behaviour in a much earlier state as it is
possible by determining growth parameters such as typical length scales
or growth rates.
DY 46.72 Do 16:00 Poster D
Temperaturabhängige Dynamik von Au-Clustern auf amorphen
C-Substratfilmen — •Ralph Werner 1 , Matthias Wanner 2 ,
Dagmar Gerthsen 2 und Stefan-Sven Jester 3 — 1 Institut für
Theorie der Kondensierten Materie,Universität Karlsruhe (TH), 76128
Karlsruhe — 2 Laboratorium für Elektronenmikroskopie, Universität
Karlsruhe (TH), 76128 Karlsruhe — 3 Institut für Physikalische Chemie,
Universität Karlsruhe (TH), 76128 Karlsruhe
Kleine, deponierte Au-Cluster zeigen in verschiedener Hinsicht dynamisches
Verhalten. Im einzelnen kann man (i) unterschiedliche kristallographischen
Modifikationen (fcc-Kuboktaeder, Ikosaeder, Dekaeder), (ii)